Operational amplifiers are one class of amplifiers that are widely used building blocks in electronic circuit systems. A typical basic operational amplifier includes a differential amplifier input stage, a gain stage, and a class AB output stage comprising source and sink transistors controlling current through an output terminal. Such a building block amplifier is usually combined with a feedback network to adapt the combination to perform a desired one of many possible linear circuit functions.
As the physical size of electronic circuits gets smaller with advances in the art, the typical power supply voltage also becomes smaller; and a need arises to derive the maximum possible voltage and current output from the amplifier. In the past, attempts to achieve a maximum current range have resulted in either a low voltage output or an undue increase in the quiescent power dissipation.
In a L. A. Kaplan U.S. Pat. No. 4,380,740, cascaded current mirror amplifiers employ matched transfer characteristic master and slave transistors of different base-emitter junction areas. This arrangement achieves current gain independently of transistor beta.
O. H. Schade, Jr., U.S. Pat. No. 3,952,257, shows a current proportioning, or splitting, circuit using a current mirror amplifier (CMA) and employing transistor geometry ratios to achieve desired current splitting. Schade also arranges his circuit so that the potential at a load resistor terminal, without saturating an output transistor, can approach within one transistor base-emitter junction voltage drop (Vbe) plus one voltage drop across the collector-emitter path of a saturated, load driving transistor.
A copending, commonly owned, U.S. patent application of D. A. Spires Ser. No. 451,024, filed Dec. 20, 1982, now U.S. Pat. No. 4,454,479, entitled "Operational Amplifier with Improved Output Capability," and issued June 12, 1985, has cross-coupled CMAs and matched resistors coupled to base terminals of output transistors, of enlarged base-emitter junction area, to control their levels of conduction with respect to the output terminal. Since the output stage has essentially infinite drive capability, a current limiting stage with feedback to the amplifier second stage input prevents self-destruction in the event the load becomes shorted.